Drag/drop changes

* Increase the opacity of the drag thumbnail a bit
* Properly present window-local drag locations; this fixes drags into
  the status bar
* Pass the current drag touch point in DRAG_STARTED. This may
  be reconsidered, but might conceivably be useful to someone.

Change-Id: I77690e82e9831ed6be412990c1c9da92e700a501